The invention relates to a method for increasing the bonding strength between grains in a subsurface formation zone, which zone is situated adjacent to a well penetrating said formation.
The invention relates in particular to the above-mentioned method wherein silicon polyhalide in a dry inert carrier fluid (gas or liquid) is passed into a formation zone having a water-wet pore space wall and wherein the silicon polyhalide has a water reactivity that is substantially equal to that of silicon tetrahalide.
Subsurface formations containing fluids, such as hydrocarbon fluids (oil and/or gas) and/or water, being recovered via wells penetrating the formations, often comprise layers of unconsolidated or incompletely consolidated formation grains. Since the bonding strength between the grains is small (or even nil) these grains (hereinafter also referred to as sand or sand particles) will be torn from the layers by fluid flowing through the pore space of the layers to the well. These grains are likely to obstruct the fluid passage in the well and/or the surface equipment communicating with the well. The tubing in the well and the surface equipment may even be locally damaged by the erosive action of the sand carried by the fluid streams that after having passed through the perforations of the tubing impinge on the inner surfaces of tubing and further production equipment.
The use of silicon polyhalide for increasing the bonding strength between the grains of such incompetent formation layers is very attractive since this type of consolidating agent can be handled in the field in a relatively simple manner. The agent may be used in vapor phase in a dry carrier gas, and is then applied in formations wherein at least the pore space of the formation part or zone to be treated has water-wet walls and is filled with gas. This gas may be the gas that is originally present in the formation, or gas that has been injected into the formation to sweep the liquid present in the pore space of the formation part to be treated to a predetermined distance from the well.
The consolidating treatment by means of a gaseous silicon polyhalide in a carrier gas has been extensively described in Applicant's British Pat. No. 1,536,209.
Also, silicon polyhalide dissolved in a suitable non-polar solvent such as crude oil or kerosene, may be used as a consolidating agent for consolidating a formation having the pore space thereof filled with crude oil. The walls of the pore space should either be water-wet, or be made water-wet prior to injecting the silicon polyhalide, U.S. Pat. No. 3,055,426 describes such a method.
Silicon tetrachloride, silicon hexacloride, silicon octachloride, as well as silicon polyfluoride have been found useful consolidating agents for consolidating incompetent formation parts surrounding a well. The silicon polyhalide hydrolyzes with water adhering to the walls of the pore space of the formation parts to be consolidated, thereby forming an amorphous silica that bonds the formation grains together with retention of permeability of the formation.
When the silicon polyhalide is used in combination with a dry carrier gas, the mixing of the silicon polyhalide with the gas may either take place at the surface of the earth at or near the well entrance, or in the well at a level near the formation part to be consolidated. In the latter case the carrier gas and the silicon polyhalide are supplied to the said level through separate passageways in the well. The silicon polyhalide may be passed either in the gaseous phase or the liquid phase to the level of the formation part to be treated. When in the liquid phase, the silicon polyhalide is sprayed (such as atomized by a spray nozzle) into the carrier gas prior to entering the pore space of the formation part to be treated.
The silicon polyhalide after having entered the pore space reacts with the water adhering to the pore space wall. If SiCl.sub.4 is used as a consolidating agent, the reaction is as follows: ##STR1## (a) either in gas phase or dissolved in a non-polar solvent; (b) liquid;
(c) amorphous; PA1 (d) gas.
Now in certain wells, the presence of the hydrochloric acid will form a problem. The acid initially remains in the pore space of the formation, in part mixed with the injected inert carrier gas or carrier liquid, and in part dissolved in the remaining connate water. The acid is subsequently displaced from the formation into the well during the subsequent production period wherein not only hydrocarbon fluids, but also formation water may flow through the pore space to the well. Since the amount of HCl is appreciable, undesired corrosion of the metal components of the well equipment will take place which requires early replacement of such components.
It has been suggested earlier in U.S. Pat. Nos. 2,808,886 and 4,061,191 to neutralize the acid by injection of ammonia. However, gaseous ammonia and gaseous HCl form a precipitate of ammonium chloride during their reaction, which decreases the permeability of the treated formation zone, which is undesirable since this forms an increased flow resistance to the flow of hydrocarbon fluids that are to be produced from the formation via the consolidated formation zone.
An object of the present invention is a simple and reliable acid-neutralization step aimed to remove acid from the formation zone treated by a silicon polyhalide to increase the bonding strength between the grains of the said zone.